linux-kernel - [PATCH 1/2] KVM: Optimize kvm_make_vcpus_request

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Date:   Fri, 20 Aug 2021 14:43:53 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] KVM: Optimize kvm_make_vcpus_request_mask() a bit

Iterating over set bits in 'vcpu_bitmap' should be faster than going
through all vCPUs, especially when just a few bits are set.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 virt/kvm/kvm_main.c | 49 +++++++++++++++++++++++++++++----------------
 1 file changed, 32 insertions(+), 17 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 3e67c93ca403..0f873c5ed538 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -257,34 +257,49 @@ static inline bool kvm_kick_many_cpus(const struct cpumask *cpus, bool wait)
 	return true;
 }
 
+static void kvm_make_vcpu_request(struct kvm *kvm, struct kvm_vcpu *vcpu,
+				  unsigned int req, cpumask_var_t tmp)
+{
+	int cpu = vcpu->cpu;
+
+	kvm_make_request(req, vcpu);
+
+	if (!(req & KVM_REQUEST_NO_WAKEUP) && kvm_vcpu_wake_up(vcpu))
+		return;
+
+	if (tmp != NULL && cpu != -1 && cpu != raw_smp_processor_id() &&
+	    kvm_request_needs_ipi(vcpu, req))
+		__cpumask_set_cpu(cpu, tmp);
+}
+
 bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
 				 struct kvm_vcpu *except,
 				 unsigned long *vcpu_bitmap, cpumask_var_t tmp)
 {
-	int i, cpu, me;
+	int i;
 	struct kvm_vcpu *vcpu;
 	bool called;
 
-	me = get_cpu();
-
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if ((vcpu_bitmap && !test_bit(i, vcpu_bitmap)) ||
-		    vcpu == except)
-			continue;
-
-		kvm_make_request(req, vcpu);
-		cpu = vcpu->cpu;
-
-		if (!(req & KVM_REQUEST_NO_WAKEUP) && kvm_vcpu_wake_up(vcpu))
-			continue;
+	preempt_disable();
 
-		if (tmp != NULL && cpu != -1 && cpu != me &&
-		    kvm_request_needs_ipi(vcpu, req))
-			__cpumask_set_cpu(cpu, tmp);
+	if (likely(vcpu_bitmap)) {
+		for_each_set_bit(i, vcpu_bitmap, KVM_MAX_VCPUS) {
+			vcpu = kvm_get_vcpu(kvm, i);
+			if (!vcpu || vcpu == except)
+				continue;
+			kvm_make_vcpu_request(kvm, vcpu, req, tmp);
+		}
+	} else {
+		kvm_for_each_vcpu(i, vcpu, kvm) {
+			if (vcpu == except)
+				continue;
+			kvm_make_vcpu_request(kvm, vcpu, req, tmp);
+		}
 	}
 
 	called = kvm_kick_many_cpus(tmp, !!(req & KVM_REQUEST_WAIT));
-	put_cpu();
+
+	preempt_enable();
 
 	return called;
 }
-- 
2.31.1